A “complicated and active” storm system was sweeping across the Eastern United States on Monday, bringing the potential for widespread thunderstorms capable of producing damaging winds, flash flooding, hail and tornadoes, forecasters said.
The system has led to tornado watches being posted across at least eight states and prompted federal offices in Washington to close early.
The unsettled weather was expected to stretch from New York to Georgia, with the highest risk in the Mid-Atlantic.
Forecasters with the Storm Prediction Center use a five-level scale to indicate the probability that severe weather will occur in a given area.
On Monday, they assigned the Washington-Baltimore area a four on that scale, a level not seen since widespread wind damage was reported across the Mid-Atlantic in June 2013.
“The atmosphere is forecast to be in a volatile state, and severe thunderstorms will likely affect much of the area,” Andrew Snyder, a National Weather Service meteorologist in Washington, said. “There is increased confidence for numerous damaging wind gusts along with the potential for locally destructive winds over 75 m.p.h.”
An area stretching from northeastern Tennessee to southern Pennsylvania could be affected by widespread severe storms, some long-lived and intense. Storms of this severity typically happen once a year, if that.
More than 18 million people were under a tornado watch issued by the Weather Service on Monday afternoon for portions of Maryland, North Carolina, Virginia, West Virginia and Washington, in effect until 9 p.m.
The service said that “a few tornadoes” were likely in those areas, as well as isolated hail up to the size of Ping-Pong balls and wind gusts of up to 75 miles per hour.
A separate tornado watch was also issued for portions of Delaware, New Jersey, New York and Pennsylvania in effect until 11 p.m.
Ahead of the storms, the United States Office of Personnel Management said federal offices in Washington would close by 3 p.m. The University of Maryland in College Park, Md., also closed early and canceled its classes on Monday.
The Storm Prediction Center said in an update on Monday afternoon that forecasters were eyeing portions of eastern Kentucky and West Virginia, where there was the “best chance for a tornado.”
The risk expands beyond the Mid-Atlantic, with more than 40 million people living within an area that forecasters believe has a moderate probability of experiencing these severe storms. The area includes major cities like Philadelphia, Atlanta and Charlotte, N.C.
The risk was lower in New York City and other parts of the Northeast, but excessive rain that could lead to flash flooding was likely, forecasters said.
Jesus Jiménez contributed reporting.